ISO in low light situations?

I have a Nikon D80 and I want to know how to take photos in low-light situations. Every time I try, the photos come out black or really super dark. I want to learn manually.

It depends how low the light is, you're obviously under-exposing.

There has to be some light for photography to be possible, but you can always up your ISO and pay attention to your meter display. If you can't get a correct reading, don't shoot, but a hand-held meter and a tripod might come in handy.

This is a trial and error situation because light levels vary all over the place. In low light I usually start with an ISO of 400. And go up from there. If you start with a very high ISO you run the risk of excessive "noise" which is the digital equivalent of grain with negative film. If depth of focus is important to you then you need to start with an f/stop of about f/8. From here you can double your ISO like going to ISO 800, then ISO 1200, etc. And yes you need to set your camera manually or you can use f/stop priority and change the ISO until you see a hand holdable shutter speed. For full manual operation assuming you do not want to use a tripod then set the camera on the shutter speed that you are confident you can hold the camera with out movement. And double/raise the ISO until you get a reasonable f/stop.

Hope this is not "clear as mud".

That's because you're on manual mode and you're not selecting the right shutter speed/aperture combination. There's a light meter inside the viewfinder that will indicate if your image will turn out too bright or too dark, so use it. Or you can use A, S, or P modes and let the camera figure it out for you (just keep an eye on shutter speed or else you'll get blurry shots).

Set to aperture priority mode, use smallest f/number, set to a high ISO value (say, ISO 1600?) then shoot. If blurry, set ISO higher. If you want manual mode, then do as above except in M mode then start at 1/30s then shoot. If too bright or blurry, select a faster shutter speed. If too dark select a slower shutter speed. A tripod would be very helpful in this case.

With a good, steady tripod, if nothing in your scene is moving, choose a low ISO like 100. You will need a long exposure to do this. The low ISO will have less noise and better dynamic range than higher ISO.